Recently the key in my Prius has gotten really worn out. When I have asked my local Toyota dealer about getting a replacement key and getting it programmed for the car, I’ve been quoted a price of ~300-500 per key. Obviously, this is crazy, so I’ve been doing some research, ordered a replacement key off eBay, and found a number of things I wanted to collect more neatly for my own future reference or for others.

I’m not done with this process – I will update this post as I learn more or make more progress.

Dismantling the key and its main parts

The key fob consists of a few main parts – a good visual overview of this is on this site. The entire block is referred to as the fob. On the top of the fob is a small sliding lever. If you slide this ~60% open, you can slide out a conventional metal key. If you slide the lever all the way open and then apply pressure and push up on the back (the no-button side) of the fob, this will slide the case off of the fob. From here you can see a sticker indicating your fob’s part number – mine is a B31EG. There are multiple Prius key fobs for different car models, some of which have smart entry and start, some which just have the remote control keyless entry feature. Below the sticker is a small, square plastic cover attached by four screws. After removing these screws, you can replace a CR2032 watch battery that powers the fob for two of the key functions described below. On the front of the fob (at least with my B31EG) the keyless entry buttons are surrounded by a sealing, rubber face. This can be peeled back and I did – however I think it probably destroys your fob. On mine, none of the buttons or keyless features were working already, anyway, so I had nothing to lose – but keep this in mind.

A thread on PriusChat.com indicates clarifies that there is no programming of the key, you program the car to accept the key. A Prius will accept up to 5 keys to be paired with it (I don’t know how to purge an old key from the Prius). The Prius key fob has four functions and each of these functions is set independently.

The physical, metal key

As mentioned above there is a physical key embedded in the fob. This is only used (on my 2004 Prius) to unlock the driver’s side door. This is a physical key so there is no “programming” to do, but obviously it needs to be cut. Online advice recommends to have a locksmith cut it rather than simply going to a local hardware store.

The passive key that starts the car

This function allows you to turn on the car by placing the key in the car’s fob slot and pressing the Power button. This works using some magic induction from the car (possibly passive RFID?) so if the battery in the key fob is dead, this will still work. I have successfully paired a replacement B31EG with my Prius by following instructions in this thread on priuschat. This was pretty easy.

[in/out*4] Insert old fob into slot and remove it 4 times in a row, then insert it a fifth time and leave it in.

[open/close*6] Open and close the driver door 6 times (leaving it closed)

[out] Remove the old fob from slot – the car is now in programming mode.

[insert new, wait] Insert the new fob and leave in the slot for a full minute. Watch the blinking red anti-theft light on the dashboard.

When it stops flashing and goes dark, your new fob is all set. Remove the new fob and you’re done.

Keyless entry (the remote control buttons)

This function enables pressing the remote control unlock/lock/panic buttons on the car and it relies on power from the fob internal battery. Instructions on this vary and seem like they may depend on the model of the fob, the model of the Prius, or both. I have not yet successfully completed this and am working on refining these steps, but the longest set of steps say to do the following:

[open] With the driver door unlocked and no fob in slot, open the driver door.

[in/out/in/out] Insert and remove the old fob into slot twice within 5 seconds

[close/open/close/open] Close and open driver door twice.

[in/out] Insert and remove old fob once.

[close/open/close/open] Close and open driver door twice again.

[in/close] Insert old fob in slot and close door.

[power/power/power] Without pressing the brake, press power button, wait a second, press it again, wait a second, press it a third time to turn car back off.

[out] Remove old fob from slot. You’ll be in “add mode” at this point. The car will lock and unlock doors to verify it is in add mode. (the precise number of times the car locks/unlocks the door is model specific – but it should do it at least once)

…from this point on, the steps are not clearly written and I don’t know what works – further editing is needed…

Press both buttons on old/new fob simultaneously for 4 flashes of the red LED on the fob.

For a silver logo fob, the timing does not matter, but it is important to hold BOTH the old and the new FOB buttons simultaneously (e.g. hold down the lock and unlock buttons of a FOB but not simultaneously do it for both FOBs holding down all 4 buttons at the same time. I did this only once so I am not 100% certain about the sequence, but this is what I think worked for me:

first press down both buttons of the NEW FOB simultaneously, car responds with locking/unlocking TWICE (not sure this step is even needed as car responding with locking/unlocking TWICE implies this is not a programmed FOB yet for this car)

then, say 1 second after the car stops locking/unlocking due to step above, press down both buttons of the OLD FOB simultaneously,

then, say after 1 second, press the LOCK button of the NEW FOB. Now, IF SUCCESSFUL car responds with locking/unlocking ONCE implying this new fob is now programmed/paired with this car.
(iv) As already mentioned in previous posts, you have 40 seconds to play around with the steps in (iii) above. I had to play around a bit since it was not clear to me if one should first use the NEW FOB or the OLD FOB in the sequence of simultaneously pressing both keys. What is clear is that one needs to use BOTH FOBs

To end this programming, open a door or put the fob in the slot.

Even though I don’t think it matters, I actually tried the smart key features after I drove the car for a minute while having the new fob in the slot. The smart key functionality worked fine.

Now select the mode by inserting the key into the Ignition Switch and turning it to the “Run” or “On” position. The programming mode is determined by the amount of times you go from “key off” to the “key on” position and back before pulling the key out.

1 time is the “Add” mode. This is used only on some models and it allows you to add a remote to the already existing remotes. The ECU confirms this by locking and unlocking the door locks automatically after you remove the key.

2 times is the second mode which will erase all previously programmed remotes and allow you to program new ones. The ECU confirms this mode by locking and unlocking the door locks twice after you remove the key

3 times is the third mode which tells you how many remotes are already programmed to the ECU. It confirms this locking and unlocking the door locks the amount of times applicable to the remotes coded. If no remotes are programmed then the ECU locks and unlocks the door locks 5 times. The ECU will hold up to 4 remotes at any one time.

Press the Lock and Unlock buttons on the remote simultaneously for 1.5 sec and then press either button by itself for 1 sec.

The ECU will perform the Lock/Unlock automatically to confirm that the 1st remote is stored by the ECU. Repeat step 7 immediately with another remote and continue until all remotes are registered.

Shut the driver’s door and try all remotes.

Smart entry and start

This is the function that enables all the “magic” functions of the car. This lets you grasp the doorknobs and will unlock the car if you have the key with you, lets you press the rubber buttons to lock the doors from outside the car (and prevents those buttons from locking the key in the car), and lets you press the “Power” button to turn the car on without putting the fob into the slot. I have not found instructions on how to program this.